Containers such as buckets and pails of unitary construction molded from plastic materials are becoming more wide spread in use due in part to the fact that they may be economically produced in large quantities. Molded plastic containers have replaced many types of previous metal constructions, however the use of containers for certain products, such as those which are toxic or flammable, require that the container be exceptionally rigid and not subject to rupture or deformation under ordinary use. In connection with this latter mentioned use, molded plastic containers have not been entirely successful in replacing metal constructions for reasons which will be discussed below. In designing a molded plastic container which is safe for use with paints, thinners and the like, it is obsolutely necessary that the lid of the container remain securely fastened to prevent leakage even when the container is roughly handled, as during shipping. Although interlocks between the container lid and the rim of the container have been designed which form a liquid tight seal under ordinary conditions, such seal may be broken and the lid may inadvertently be removed from the container if the rim exhibits flexion.
In order to prevent flexing of the container rim, circumferential reinforcement ribs have been molded integral with the rim in order to increase the hoop strength of the rim. Since it is also a requirement of these type of containers that the sidewalls thereof be free from projections, it is necessary to recess the circumferential reinforcement ribs in the sidewalls of the container; this in turn requires that the rim be of reduced inside diameter compared to the inside diameter of the sidewall of the container.
Because of the reduced rim diameter, it has been necessary in the past to produce the containers using a two step blow and injection molding process which involves melting a thermoplastic resin; forming a tube or parison from the melted resin using injection molding techniques; and, inflating the parison within a blow mold to form the container. This process is undesirable, however, since the two production steps not only require at least two molds but are also time consuming compared to a one step process.
Other types of molding processes, such as injection molding, have not been successfully employed in the past to mold containers of the type mentioned because collapsable molding cores of the type necessary for forming the interior sidewalls of the container have not heretofore been available. Known prior art collapsable cores, such as those disclosed in U.S. Pat. Nos. 3,247,548 and 3,660,001, are suitable for forming cavities in relatively shallow articles such as threaded container caps, but are not capable of forming deep cavities, such as the interior of a bucket. Collapsable cores of the type disclosed in these patents are formed from a plurality of circularly disposed, resilient molding segments circumferentially surrounding an inner core and rigidly connected to each other at one end thereof. Upon removal of the inner core, the natural resilience of the molding segments causes one end of such segments to spring radially inward, thereby collapsing the outer extremities of the molding segments to release the molded article. Since only the outer extremities of the molding segments collapse a substantial distance, these collapsable cores may not be practically employed for molding deeper cavities such as the interior of the container itself.
From the foregoing, it is apparent that there is a clear need in the art for a container of the type described above which comprises high density plastic material. It is therefore an important object of the present invention to provide a container of bucket-like configuration having a reinforced rim provided with an interior diameter less in magnitude than the interior diameter of the sidewall of the container, but yet which is exceptionally rigid and durable.
Another important object of the invention is to provide a one step process for molding a container of the type mentioned above.
Another object of the invention is to provide die apparatus suitable for injection molding plastic material to form the container mentioned above.
A still further object of the invention is to provide a die apparatus having a collapsable core formed of a plurality of core segments each being radially shiftable along the entire lengths thereof.
These and further objects will be made clear or will become apparent during the course of the following description.